1. Field of the Invention
This invention relates to a method of controlling electromagnets, and more specifically to a method of controlling current changes in, and thus magnetic fields produced by, electromagnets by using a patterned rate of current change in a manner to cause the electromagnets to function faster, more efficiently, more safely, or more economically, or in a manner in which a combination of these advantages accrue.
2. Description of Related Art
Electromagnets of various types have been used for such diverse purposes as the magnetic suspension of wind-tunnel models and the guiding of implant placement in living tissue to deliver therapy to a patient. In these and in many other applications, it has proven useful to control the power source of the electromagnet with feedback systems having characteristics designed to satisfy load requirements or system requirements, or both. Often, it would be advantageous to allow these power sources to provide rapid current changes in response to conditions imposed by or on the system that is influenced by the magnetic field of the electromagnet.
Superconducting magnets made of coils of superconducting material (especially the new, high-temperature type of superconductors) present special problems when rapid current change is required. Superconducting coils have zero resistance, except for parasitic resistance that includes their external leads and connections to the powering device. Therefore, these coils have a long time constant for the increase of current, given by the value of L/R, where L is coil inductance and R is the effective value of the various sources of parasitic resistance. Because R is very low, superconducting coils act as almost pure inductors, and have very long time constants.
In a typical prior art use of superconducting magnets (such as described in McNeil et al., "Characteristics of an Improved Magnet-Implant Guidance System," pp. 802-808, IEEE Trans. Biomed. Eng., Vol. 42, No. 8, August 1995, which is hereby incorporated by reference in its entirety), a constant voltage V is applied to the superconducting coil. Thus, a good approximation of the ramping rate of current through the coil is given by assuming the coil acts as a pure inductance, i.e., dI/dt=V/L. Unless the applied voltage V is very large, or L is very small, the rate of current increase is low. Large coils can thus require hours, or even days, to power up, which is at least inconvenient and may be intolerable in some applications. Attempts to increase the ramping rate by using moderately high values of constant voltage V can result in quenching (i.e., loss of conductivity) of the superconducting coil, which can be hazardous to equipment. In addition, a high voltage could be established in the coil in some transient circumstances, possibly exceeding the breakdown voltage of the coolant (such as helium, or helium gas bubbles, which may also be present) and causing serious coil damage.
Coils can quench as a result of a combination of two factors: (a) an excessively high magnetic field resulting from excessive current in the coil, and (b) eddy-current heating resulting from current changes being applied to the coil too rapidly. (High fields and eddy current heating can result either from current in the coil itself, or from nearby sources of magnetic fields. The effect of nearby sources of magnetic fields is not hereafter explicitly taken into account, because these effects are of secondary significance in most applications. There are also other factors that are difficult to model mathematically that are known to contribute to the tendency of a coil to quench--e.g., liquid helium proximity to the coil, and the thermal capacity and conductivity of the coil bobbin and support structure. Thus, the quenching behavior of any particular coil can usually only be determined by experimentation and modeled approximately.) It is well-known that a given superconducting coil will quench at a lower current if the ramping rate (i.e., first derivative of the current) is high, and also that it will quench at some lower ramping rate if the field (i.e., the current itself) is high. Taken together, these effects combine so that there is a tendency for quenching to occur at a roughly constant value of the magnitude of the product of the current and its rate of change (i.e., the first derivative of the current). It would be desirable to provide a faster method of ramping current in superconducting coils that avoids the problem of quenching.
It would also be desirable if a high voltage magnitude, when used to ramp the coil, could be maintained only up to the arcing limit of the coil while the magnitude of current in the coil is low, and a lower magnitude of voltage applied as the magnitude of current increases. Such a method would allow rapid ramping of current while avoiding the risk of damage caused by insulator breakdown.
The limitations of superconducting coils when used with most present current ramping methods are so severe as to prevent the use of such coils in traditional servo systems or in manners in which limited ramp time is important. In other applications, designers have been forced to limit the rate of change of current to a value found to avoid quenching in worst case conditions, and to limit the current itself to a value found to avoid quenching. Alternately, if faster ramping is needed, coil designers have had to undertake heroic steps to make the coil less vulnerable to ramp-time quench, although these steps can never completely avoid the possibility of quench. Consequently, the use of superconducting coils in dynamic devices has been almost completely ruled out.
More specifically, the temperature T and field H.sub.c at which a superconducting wire will change phase is given approximately by the equation EQU H.sub.c =H.sub.o [1-(T/T.sub.c).sup.2 ] (1)
for an especially simple superconducting material, and by modified versions of the equation for others. Here, H.sub.c is the critical field above which the material will change phase, H.sub.o is the critical field at absolute zero temperature, and T.sub.c is the critical temperature above which the material will not be superconducting at any field value. Thus, the tendency of a coil to quench will depend on its own current (an I-dependence), in addition to fields created by external sources. In summary, coils can quench from high field effects or eddy-current heating from ramping too fast. The high field effects or eddy-current heating can be caused by the coil itself, or by other nearby sources.
As described above, power supplies of fixed voltage have been used in the prior art to provide linear ramping of a single superconducting coil. Standard servo methods have been used for ramping non-superconducting coils. In the less common case of multi-coil systems, special temporal relationships have been used in more complex ramping systems, such as in allowed U.S. patent application Ser. No. 08/280,124, filed Jul. 25, 1994, and in currently pending U.S. patent application Ser. No. 08/682,867, filed Jul. 8, 1996. The specifications of both of these patent applications are incorporated herein in their entirety. In one use of superconducting electromagnets, a six-coil helmet (such as described in McNeil et al., "Functional Design Features and Initial Performance Characteristics of a Magnetic-Implant Guidance System for Stereotactic Neurosurgery," pp. 793-801, IEEE Trans. Biomed. Eng., Vol. 42, No. 8, August 1995 which is hereby incorporated by reference in its entirety) was designed to move a magnetic implant object ("seed") around within a human brain (or some other part of a human body) to deliver therapies such as selective hyperthermia, radioactivity, chemicals, or other substances.
A control method described in McNeil et al., "Characteristics of an Improved Magnetic-Implant Guidance System," pp. 802-808, IEEE Trans. Biomed, Eng., Vol. 42, No. 8, August 1995 (which is hereby incorporated by reference in its entirety) partially avoided the usual limitations encountered in ramping a superconducting coil by controlling coils in pairs in which the two members of each coil pair operate in a different dynamic manner to provide stepwise, impulsive forces on the seed. The current through a main, pulling coil is kept at a subthreshold level (i.e., below a value required to move a seed in a brain, in the disclosed application) throughout each step. Current in a much closer partner coil (a "boost" or "push-pull" coil) is then ramped up a small amount to apply an opposite field by additive gradient (of magnetic field) on the seed. Because it is closer to the seed, the gradient caused by the closer coil can be much greater without reversing the seed direction, so a large pushing impulsive force could be applied. The push-pull coil then has its gradient reversed to pull, for a short time, against the main pulling coil to halt the seed movement and reestablish a stable position at rest. The gradient of the push-pull coil falls off rapidly with distance from the coil, approximately as the fourth power of the distance. Therefore, the close coil can effect great changes in force at the seed position with small changes in a small current. Modifications to this technique can be used in cases in which the main, pulling coil is not at a much greater distance from the seed than the partner coil. This technique and its modifications can be accomplished with constant voltage ramping in accordance with the prior art, but limitations inherent in constant voltage ramping considerably restrict the speed of the motion of the seed. These limitations come about because the maximum voltage magnitude that can be applied can be no greater than that which results in quenching when applied simultaneously with the maximum allowable current in the coil. By restricting the maximum voltage magnitude, the rate at which coil current changes can be accomplished, and hence the rate at which changes in the resulting magnetic field can be made that effect movement of the seed, is limited.
It would therefore be advantageous if a ramping method were available for coils that could minimize ramping time between two current levels, while avoiding quenching and arcing. It would be particularly advantageous if this system were applicable to systems employing multiple coils to apply force and/or direction to a magnetic seed.